This invention relates to a multi-pole circuit breaker comprising current limiting devices in which, when a large current such as short-circuit current flows, the contactors are operated to open the circuits to increase the arc voltages before the ordinary switching mechanism is operated. The current limiting interruption is carried out with the subsequent circuit opening operation of the switching mechanism.
A current limiting device employing electromagnetic repulsion force utilizes an electromagnetic force generated by currents flowing in two parallel conductors in opposite directions. An ordinary arrangement of the current limiting device is as described below. The movable contactor and the stationary contactor are arranged in parallel with each other so that current flows in the contactors in the opposite directions. As soon as a large current flows, one or both of the movable contactor and the stationary contactor are separated from each other against the elastic force of a spring providing a contact pressure by utilizing an electromagnetic force generated between the contactor, before this prior art circuit opening operation is carried out by the switching mechanism. Alternatively, by utilizing the electromagnetic force, a locking device provided for the movable contactor or the stationary contactor is unlocked to quickly open the circuit. The interruption operation is accomplished with the subsequent ordinary circuit opening operation.
In a conventional multi-pole circuit breaker, a current limiting device of this type is provided for each of the poles. That is, the number of current limiting devices is equal to the number of poles. Accordingly, the conventional multi-pole circuit breaker is larger in size and higher in cost than a non-current-limiting type multi-pole circuit breaker having no current limiting device. As is well known in the art, when a three-phase short-circuit occurs, currents flow differently according to the phases, depending on the short-circuited phase, and progress through transient conditions. In the conventional circuit breaker, current limiting devices are provided for all of the poles, respectively, to operate independently. Accordingly, at the time of interruption, the current limiting device for a phase highest in current increase factor (di/dt) is first operated to open the circuit. Therefore, the current of a phase smaller in current increase factor, i.e. the minor loop current is not always interrupted (the first phase interruption). Depending on the short-circuit current magnitude and the short-circuit phase, the major loop current high in current increase factor may be subjected to first phase interruption. If the voltage is high, the interruption is severe.